Recently, as various types of nanostructures having a size of nanometers have started to be researched and applied extensively to the fields of semiconductor devices, energy, displays, biological technologies, medical services and the like, nanoscale-resolution imaging technologies or nanoscale electromagnetic wave focusing technologies have become very important issues.
Among such technologies, particularly, X-ray microscopy can provide accurate nanoscale chemical, structural and morphological information due to the inherent characteristics thereof such as high transmittance, short wavelength and the like, so it will be expected that it can provide complimentary material information that can not be provided by an optical microscope or an electron microscope.
According to the Rayleigh criterion R=0.5λ/NA (here, R is resolution, λ is wavelength, and NA is numeric aperture), X-rays and γ-rays can be theoretically focused in a size similar to the wavelength thereof. However, to date, a conventional X-ray/γ-ray focusing optical system has not reached the theoretical X-ray/γ-ray focusing size because it was difficult to manufacture this optical system that can focus X-rays and g-rays, so research into whether any type of optical system is theoretically effective has been actively carried out.
An X-ray focusing optical system for focusing X-rays having a refractive index of less than 1.0 and a very short wavelength region requires far stricter conditions than general optical systems. That is, in the X-ray focusing optical system, X-rays are refracted or reflected at a very small angle because their interaction with a material is very weak. In other words, in the X-ray focusing optical system, it is difficult to obtain a large numeric aperture, and the focusing efficiency of X-rays and γ-rays becomes very low, so there are many difficulties in obtaining nanometer-scale X-ray and γ-ray beams. All over the world, there have been various attempts to improve X-ray/γ-ray focusing optical systems. Among them, as typical examples thereof, there are a Fresnel Zone Plate and a Multilayer Laue Lens.
As shown in FIG. 1, a Fresnel Zone Plate has a structure in which transparent and opaque circular rings are repeatedly and alternately arranged, and is an optical system for focusing light rays using a diffraction phenomenon of electromagnetic waves. A general Fresnel Zone Plate has a circular lattice shape, and includes transparent layers through which X-rays and γ-rays are transmitted and opaque layers which serves to change a phase, the transparent layers and the opaque layers being alternately arranged. In this case, X-rays and γ-rays diffracted by each of the layers converge into a focus based on a principle of constructive interference occurring at specific points. Each of the layers is disposed at the position which is away from the origin by the distance calculated by Fresnel Equation below:r2n=nλf+n2λ2/4
Here, rn is a position of an nth layer, and f is a focal distance. The second term may be omitted when nλ<<f. The thickness Δr of the outermost layer is rn-rn-1. The resolution of a Fresnel Zone Plate is approximately equal to the thickness Δr of the outermost layer (R=1.22Δr when the Fresnel Zone Plate is a circular zone plate, and R=Δr when the Fresnel Zone Plate is a linear zone plate). Further, the optical contrast between a light-transmitting layer and a light absorbing layer, the depth of light transmission (the size of an optical system in a direction of beam progressing), the accuracy of each layer, the original size of incident beam, coherences, and the like become important factors for determining the resolution of the Fresnel Zone Plate.
As conventional methods of manufacturing a Fresnel Zone Plate, there are a method of manufacturing a Fresnel Zone Plate using electron beam lithography and an electrochemical technology and a method of manufacturing a Fresnel Zone Plate using a sputtering-slicing process.
The focusing efficiency of a zone plate is influenced by the thickness of the outermost layer, raw materials, X-ray and γ-ray energy, and the like. Particularly, the focusing efficiency thereof greatly depends on an aspect ratio (ratio of the thickness of the outermost layer to the depth of light transmission).
First, as shown in FIG. 2, in the method of manufacturing a zone plate using electron beam lithography and an electrochemical technology, in order to obtain a large aspect ratio, a Si3N4 membrane is coated with a photosensitizing agent (PMMA, ZEP or the like) (ST21), a zone plate pattern is formed on the Si3N4 membrane coated with the photosensitizing agent by electron beam lithography (ST22), and then the zone plate pattern is deposited with gold (Au) by electroplating to complete a zone plate (ST23).
This method is an existing method of manufacturing an X-ray/γ-ray focusing optical system in the highest performance (at a level of 20 nm), and is a bottom-up growing method. The line width of the outermost layer of the zone plate manufactured by this method greatly depends on the intensity of a focused electron beam.
Recently, with the advancement of technology, it is known that an electron beam can be focused to a level of about 1 nm or less. However, the thickness of the outermost layer of the zone plate manufactured by this method is currently at least 15 nm (in this to case, the depth of light transmission is about 200 nm) because of the resolution of a photosensitizing agent, the stability of equipment (vibration or the like), the scattering of electrons in an electron beam process, or the like.
Further, when this method is used, light transmission depth determining a focusing efficiency is substantially limited to 1 μm or less, so the focusing efficiency of a Fresnel zone plate used in light X-rays and γ-rays is very low (several %), and as shown in FIG. 3, a flat Fresnel zone plate ((a) of FIG. 3) can be manufactured, but an ideal Fresnel zone plate ((b) of FIG. 3) cannot be manufactured.
Next, as shown in FIG. 4, in the method of manufacturing a zone plate using a sputtering-slicing process, the zone plate is manufactured by the steps of: rotating a wire substrate (ST31); depositing the rotating wire substrate with an opaque material by sputtering (ST32); depositing the rotating wire substrate with a transparent material by sputtering (ST33); alternately repeating the steps ST32 and ST33 (ST34); and slicing the wire substrate alternately deposited with the opaque and transparent materials. This method can manufacture a structure having a high aspect ratio, but it is difficult to finely control a structure and to focus X-rays and γ-rays at a high resolution of 20 nm or less.
Second, as shown in FIG. 5, a multilayer Laue lens (MLL) is manufactured by depositing, cutting and grinding a thin film, not by photolithography.
FIG. 6 shows a conventional method of manufacturing a multilayer Laue lens (MLL) using physical vapor deposition (PVD). In this method, the multilayer Laue lens (MLL) is manufactured by the steps of: depositing a flat silicon wafer with several hundreds or several thousands of thin film layers to predetermined thickness by sputtering (ST41), cutting the flat silicon wafer deposited with the thin film layers (ST42); grinding the cut flat silicon wafers (ST43); and attaching the grinded flat silicon wafers to each other (ST44). When X-rays and γ-rays are applied to the MLL, X-ray and γ-ray beams are focused by diffraction. In the deposition of the flat silicon wafer, since the thinnest outermost layer is first deposited, defects in the important portion thereof determining a light focusing effect must be minimized. One zone plate is obtained by facing two multilayer thin film sections each other. In this case, when an MLL (slanted MLL) is rotated by a Bragg diffraction angle (θB=0.05-0.3°), a focusing efficiency higher than that of a flat MLL can be obtained. Further, when beam transmission depth is determined by a cutting-grinding process, a very high aspect ratio (the ratio of outermost layer thickness of 5 nm to beam transmission depth of 10 μm is 2000) can be obtained, so the focusing efficiency of the MLL is several ten times higher than that of a general Fresnel zone plate. Basically, since an MLL is a one-dimensional focusing lens, two MLLs must intersect each other.
An MLL is identical to a Fresnel zone plate in a conception, but is a novel lens that can overcome the problems associated with a Fresnel zone plate because its manufacturing method is excellent. However, this MLL manufacturing method is problematic in that an MLL is fabricated on a flat surface, so X-rays and γ-rays can be focused in only one direction, and X-rays and γ-rays cannot be focused in a circular shape.